1. Field of the Invention
The present invention relates to a belt drive control method and a belt-drive control device that controls drive of an endless belt wound around rollers, and an image forming apparatus that includes the belt-drive control device.
2. Description of the Related Art
An image forming apparatus includes a belt such as a photosensitive belt, an intermediate transfer belt, and a paper conveyor belt. In such image forming apparatus, it is essential to control drive of the belt with high accuracy to obtain high-quality images. Particularly, for a tandem image forming apparatus of a direct transfer system that is excellent in an image forming speed and suitable for a reduction in a size, it is required to control driving of a conveyor belt for conveying a recording sheet with high accuracy. In this type of image forming apparatus, the recording sheet is conveyed by the conveyor belt and sequentially passed through a plurality of image forming units that are arranged along a direction of conveyance of the recording sheet. Single-color images of different colors are formed in each of the image forming units to be superimposed one another on the recording sheet. Thus, a color image is formed on the recording sheet.
An example of the tandem image forming apparatus according to an electrophotographic system is explained below with reference to FIG. 23. In the image forming apparatus, for example, image forming units 18Y, 18M, 18C, and 18K that form single-color images of yellow, magenta, cyan, and black respectively are sequentially arranged in the direction of conveyance of a recording sheet. Electrostatic latent images are formed on surfaces of photosensitive drums 40Y, 40M, 40C, and 40K by a laser exposure unit (not shown). The electrostatic latent images are developed by image forming units 18Y, 18M, 18C, and 18Y, respectively, to form toner images (visual images). The toner images are sequentially transferred onto a recording sheet (not shown). The recording sheet is caused to adhere to a conveyor belt 210 by an electrostatic force so that the recording sheet is conveyed on the conveyor belt. The toner images are superimposed one another on the recording sheet. Then, the toner is melted and compression-bonded by a fixing device 25 to form a color image on the recording sheet. The conveyor belt 210 is laid over a driving roller 215 and a driven roller 214 that are arranged in parallel to each other, with an appropriate tension. The driving roller 215 is driven to rotate at a predetermined rotational speed. The conveyor belt 210 moves endlessly at a predetermined speed following the rotation of the driving roller 215. The recording sheet is supplied to the conveyor belt 210 on a side on which the image forming units 18Y, 18M, 18C, and 18K are arranged by a sheet feeding mechanism at predetermined timing. The recording sheet moves at a speed identical to the moving speed of the conveyor belt 210 to sequentially pass the image forming units.
In such an image forming apparatus, unless the moving speed of the recording sheet, that is, the moving speed of the conveyor belt 210 is maintained at a fixed speed, color drift occurs. The color drift is caused when transfer positions of the single-color images to be superimposed one another on the recording sheet are relatively shifted from one another. When the color drift occurs, for example, a fine line image formed by superimposing plural images of different colors one another appears blurred, or a white void occurs around an outline of a black character image that is formed in a background image formed by superimposing plural images of different colors.
FIG. 24 illustrates a tandem image forming apparatus that adopts an intermediate transfer system. In the intermediate transfer system, single-color images formed on the surfaces of the photosensitive drums 40Y, 40M, 40C, and 40K of the image forming units 18K, 18M, 18C, and 18K are sequentially transferred onto an intermediate transfer belt 10. The single-color images are thus superimposed one another on the intermediate transfer belt 10, and then, collectively transferred onto the recording sheet. Also in this apparatus, unless a moving speed of the intermediate transfer belt 10 is maintained at a constant speed, color drift occurs.
In an image forming apparatus in which a belt is applied as a recording-medium transfer belt or an image carrier, if the belt does not rotate at a constant speed, banding occurs during image transfer. The banding is a phenomenon in which unevenness of image concentrations occurs. An image portion that is transferred onto the belt when the belt moving speed is relatively high appears stretched to be longer in a direction of a circumference of the belt than the original image. Conversely, an image portion that is transferred onto the belt when the belt moving speed is relatively low appears shrunk to be shorter in the direction of the circumference than the original image. Consequently, the image portion stretched has a low concentration and the image portion shrunk has a high concentration. As a result, unevenness of image concentrations occurs in the direction of the circumference. Such a problem is significant when a light-colored image of a single color is formed.
Thus, in image forming apparatuses, it is essential to accurately control driving of an endless belt, such as a photosensitive belt, an intermediate transfer belt, and a conveyor belt. One approach is to detect an angular displacement or a rotation angular speed of a driven roller, over which the endless belt is laid, and control rotation of a driving roller based on a result of detection. See, for example, Japanese Patent Application Laid-open No. S63-300248 and Japanese Patent No. 3186090. An encoder is attached to the driven roller and it detects an angular displacement or a rotational speed of the driven roller. The speed of the endless belt is subjected to feedback control based on a detection signal from the encoder. The speed of the endless belt is maintained to a constant value by maintaining a rotation angular speed of the driven roller constant. However, an angular displacement of rotational speed of rollers can fluctuate due to various factors such as eccentricity of the driven roller itself or eccentricity of attachment of the encoder to the driven roller.
A solution has been disclosed in Japanese Patent Application Laid-open Nos. H9-267946, H11-202576, and 2000-47547. An image forming apparatus disclosed in Japanese Patent Application Laid-open No. H9-267946 includes a filter unit to eliminate a rotation frequency component (a detection error) of the encoder roller from a detection signal of the detecting unit and controls moving speed of the endless belt based on the detection signal filtered by the filter unit.
An image forming apparatus disclosed in Japanese Patent Application Laid-open No. H11-202576 controls the driving of the endless belt as described below. The image forming apparatus subjects a detection signal of the detecting unit to frequency resolution, reads a rotation frequency of the encoder roller from the detection signal subjected to the frequency resolution, and extracts a magnitude (a level) and a phase of an eccentricity component of the encoder roller from the rotation frequency of the encoder roller read and the detection signal subjected to the frequency resolution. Then, the image forming apparatus eliminates extracted eccentricity component from the detection signal and controls a moving speed of the endless belt based on the signal from which the eccentricity component is eliminated.
In an image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2000-47547, a driving roller and an encoder roller having diameters different from each other are provided. The driving roller is driven to rotate at a constant speed. Angular speed information of the encoder roller is obtained for at least one rotation period of the driving roller by a detecting unit. The angular speed information obtained is divided by a half rotation period of the driving roller. A former half and a latter half of the period are added to offset a speed fluctuation component due to eccentricity of the driving roller from the angular speed information. A detection error due to eccentricity of the encoder roller is obtained from the angular speed information from which the speed fluctuation component due to eccentricity of the driving roller is offset. At the time of image formation, the moving speed of the endless belt is controlled based on differential data of the angular speed information detected by the detecting unit and the detection error obtained.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. H9-267946, when filter processing by the filter unit is performed digitally, since a large amount of calculation is required, processing time is long. In addition, to perform such arithmetic processing, expensive hardware is necessary. When the filter processing is performed analogically, it is necessary to perform digital-analog conversion. Since a conversion error occurs at the time of the conversion, accurate rotational speed fluctuation of the encoder roller is difficult to be obtained.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. H11-202576, since a large amount of calculation is required for subjecting a frequency of a detection signal to frequency resolution, processing time is also long. It is also necessary to use expensive hardware to perform arithmetic processing described above.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2000-47547, it is possible to control an amount of calculation for extracting a detection error from a detection signal. However, since it is necessary to store detection signals as a data string for one or more rotation periods of the driving roller, a storing unit with a large capacity is required. The fluctuation in a rotational speed of the encoder roller further includes, besides the fluctuation component caused by eccentricity of the driving roller and the fluctuation component caused by eccentricity of the encoder roller, a fluctuation component caused by a slip of the driving roller and the belt. Thus, detection error data to be extracted includes other fluctuation components such as the fluctuation component caused by a slip of the driving roller and the belt in addition to the rotational speed fluctuation due to eccentricity of the driving roller. Therefore, even if a moving speed of the endless belt is controlled based on the differential data of the angular speed information detected by the detecting unit and the extracted detection error, it is impossible to convey the belt at a constant speed.
Moreover, in the image forming apparatuses disclosed in Japanese Patent Application Laid-open Nos. H9-267946, H11-202576, and 2000-47547, to accurately calculate fluctuation in a rotational speed of the encoder roller, a rotary encoder with high resolution is required. Therefore, the image forming apparatus becomes expensive.